Tumor cells shed from primary tumor sites circulating in the blood stream are the so-called circulating tumor cells (CTCs). Many clinical reports have shown a strong correlation between disease development and the number of circulating tumor cells in the peripheral blood of cancer patients. Therefore, the identification, enumeration, and characterization of CTCs may provide a minimally invasive method for assessing the cancer status of patients and for prescribing personalized anticancer therapies (X. Fan et al., 2015). As metastatic tissue is often inaccessible and substantial heterogeneity has been shown between both the primary tumor and its metastases as well as between different metastases of an individual patient (Stoecklein & Klein, 2010; Almendro et al, 2014), molecular CTC analysis can serve as an easily accessible liquid biopsy for metastatic diseases.
Isolation and detection of CTCs remain a challenge in clinical oncology because of their extreme rarity, at only 1 CTC/109 blood cells (Lin et al., 2010; Ross et al., 1993). In the past few decades, numerous methods have been developed to separate CTCs from whole blood, but current technologies have shown many limits. First of all, CTCs undergo mechanical stress and fixation steps that hamper mRNA profiling and reduce genome integrity. In addition, immuno-based enrichment methods rely on the expression of EpCAM epithelial marker by CTCs, consequently lacking the detection of EpCAM negative CTC subpopulation. However, tumor cells that are able to enter the blood stream may undergo an epithelial-mesenchymal transition, resulting in the down-regulation or negative expression of EpCAM (Kölbl, 2016).